Molecular technique 'lights up' Alzheimer's origins

Researchers at Rice University (Houston, TX) may have found a way to simplify Alzheimer's disease detection—a breakthrough that may lead to better medications for its treatment as well. Detailed in a new paper published in the Journal of the American Chemical Society, the research team, led by Rice bioengineer Angel Martí, tested metallic molecules they created that naturally attach themselves to a collection of beta amyloid proteins called fibrils, which form plaques in the brains of those with Alzheimer's. When their molecules, complexes of dipyridophenazine ruthenium, latch onto amyloid fibrils, their photoluminescence increases 50-fold.

Nathan Cook, a Rice graduate student and the paper's lead author, began studying beta amyloids when he joined Martí's lab, with the goal of finding a way to dissolve amyloid fibrils in Alzheimer's patients. He then realized that ruthenium complexes had a distinctive ability to luminesce when combined in a solution with amyloid fibrils.

Ruthenium-based molecules added to the amyloid monomers do not fluoresce, Cook says. But once the amyloids begin to aggregate into fibrils that resemble "microscopic strands of spaghetti," hydrophobic parts of the metal complex are naturally drawn to them. "The microenvironment around the aggregated peptide changes and flips the switch" that allows the metallic complexes to light up when excited by a spectroscope, he says.

Click to Enlarge

Amyloid fibrils like those magnified here 12,000X are thought to be the cause of plaques in the brains of Alzheimer's disease patients. Rice University researchers have created a metallic molecule that becomes strongly photoluminescent when it attaches to fibrils. (Image courtesy of Nathan Cook, Rice University)

The metal complexes created by Rice offer a Stokes shift of 180 nm, says Martí. "We excite at 440 and detect in almost the near-infrared range, at 620 nm," an advantage when screening drugs to slow amyloid fibril growth, he says. "Some of these drugs are also fluorescent and can obscure the fluorescence of ThT [Thioflavin T dyes], making assays unreliable," he adds.

Cook also exploited the metallic's long-lived fluorescence by "time gating" spectroscopic assays. "We specifically took the values only from 300 to 700 ns after excitation," he says. "At that point, all of the fluorescent media have pretty much disappeared, except for ours. The exciting part of this experiment is that traditional probes primarily measure fluorescence in two dimensions: intensity and wavelength. We have demonstrated that we can add a third dimension—time—to enhance the resolution of a fluorescent assay."

The researchers said their complexes could be fitting partners in a new technique called fluorescence lifetime imaging microscopy (FLIM), which discriminates microenvironments based on the length of a particle's fluorescence rather than its wavelength.

The technique may make its way to treatment for other diseases, such as Parkinson's, possibly combining the ruthenium complex's ability to target fibrils and other molecules' potential to dissolve them in the brain.

The Welch Foundation (also in Houston, TX) supported the research.

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Posted by Lee Mather

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